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On the drifts exhibited by cryogenically cooled InSb infrared filtered detectors
and their importance to the ATSR-2 and Landsat-5 earth observation missions.
E. Theocharous
Optical Radiation Team, Quality of Life Division,
National Physical Laboratory, TW11 0LW, UK
Phone: +44 (0)20 8943 6977 Fax: +44 (0)20 8943 6935
e-mail: e.theo@npl.co.uk
Abstract
The spectral responsivity of commercially available InSb detectors with low-pass cold
filters attached on their cold shields for optimum operation in the 1.6 µm to 2.6 µm
wavelength range was observed to drift slowly with time. These drifts were shown to
arise due to a thin film of water-ice depositing on the cold low-pass filters mounted on
the cold shields of the detectors. The temporal characteristics of these drifts were
shown to be strongly dependent on wavelength. A model is proposed for the
behaviour of the water present in the dewar-vacuum which can explain and predict the
temporal characteristics of the observed drifts for all wavelengths. These observations
are particularly relevant to space instruments which use cryogenically-cooled infrared
filter radiometers for earth observation. The temporal profile of drifts observed in
missions like Landsat-5 is identical to that observed in cryogenically cooled filtered
InSb detectors during laboratory measurements. This work confirms that the
deposition of a thin film of a material like ice on the cold band-pass filters/windows is
therefore the most likely source of the oscillatory drifts observed in the response of
some of the channels of the ATSR-2, and Landsat-4 and Landsat-5 earth observation
missions.
Proceedings NEWRAD, 17-19 October 2005, Davos, Switzerland 167